The present invention relates to a new and distinct cultivar of a Japanese pear tree (Pyrus pyrifolia), and, particularly, to a tree bearing fruits with a yellowish-green skin, having charateristics substantially the same as those of `Osanijisseiki` (unpatented in the United States) but having a significantly strong resistance to black spot disease.
`Osanijisseiki`, which was discovered as a self-compatible spontaneous bud sport of the Japanese pear `Nijisseiki` and was released in 1979, has substantially the same characteristics as those of `Nijisseiki` except that `Osanijisseiki` is self-compatible. It has, however, a shortcoming in that `Osanijisseiki` has a high susceptibility to black spot disease, as does `Nijisseiki`. Black spot disease is one of the most important diseases of susceptible varieties, such as `Nijisseiki` and `Osanijisseiki`, and a great deal of labor is required to control it. A new cultivar of the pear fruit tree having a strong resistance to black spot disease, while also having the excellent characteristics of `Osanijisseiki` has been required for a long time.
The object of the present invention is to obtain a resistant mutant cultivar of the pear fruit tree, having a strong resistance to black spot disease while also having the excellent characteristics of `Osanijisseiki`, using irradiation of .gamma.-rays.